This invention relates to a method of forming zeolite articles, especially those usable as adsorption plates in cryopumps.
The use of zeolite material as a desiccant is well known. Zeolites are complex silicates containing aluminum and one or more other metallic elements, usually sodium, potassium or calcium. Crystals of zeolite typically have a strong affinity for water molecules and usually adsorb water in preference to any other substance. Zeolites also display a somewhat similar preference for certain types of hydrocarbons. See "Molecular Sieves" in Scientific American Magazine, Vol. 200, No. 1, pp. 85-90 (Jan. 1959).
Zeolites are usable for enhancing the pumping action of cryogenic pumps. Such pumps contain a multiplicity of panels through which cryogenic fluid circulates and are disclosed in U.S. Pat. No. 4,207,746 "Cryopump" issued to the inventor herein, the disclosure which is hereby incorporated by reference. The panels are made of a high thermal conductivity metal such as aluminum. A combination of zeolite powder and a clay binder is applied to the surface of the panel in the following manner. A screen or grid-type structure is attached to the panel surface to provide mechanical support. Zeolite powder is mixed with a clay such as kaolin and a solvent to form a slurry which is then applied to the surface of the panel by a technique such as repetitive brushing, spraying, or slip casting. The slurry coated panel is then baked in an oven to drive off the solvent and other volatiles, thereby leaving a zeolite casting affixed to the panel.
In operation a cryopump is reduced to a temperature of 25.degree. K. or lower. To be effective the surface of the zeolite casting must have comparable temperatures. The typical metallic panel and the zeolite casting each have different coefficients of expansion. These produce differential strains at the interface between the casting and the panel surface, which with repetitive use result in damage to the casting. The zeolite material will frequently spall or physically separate from the panel. Obviously, loss of material is disadvantageous; separation disrupts the conductive path between the casting and the panel thereby raising the temperature of the casting and making operation of the pump inadequate. With the present type of materials and construction, periodic inspection and repair of panels must be undertaken.